1. Field of the Invention
The present invention generally relates to the field of multiple access communications systems. More particularly, the present invention is directed to asynchronous multiple access simplex mode communications systems and methods therefor. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Related Art
In simplex mode communication systems of the related art, information can be transferred in only one direction, i.e. from one or more transmitters to one or more receivers. Some examples of such simplex mode communications systems include radio-controlled toy cars, remote data-collection systems and remote-controlled joysticks. In applications such as these, where multiple transmitters may be attempting to communicate with one of a plurality of receivers within close proximity to one another, differentiation between the various transmitter signals at the receivers is necessary to avoid signal collision and loss. Such differentiation has conventionally been accomplished, for example, by frequency division multiple access schemes wherein each transmitter/receiver pair is assigned a unique communication channel of a particular frequency band. Dividing the frequency spectrum in this manner reduces the possibility of signal confusion when two or more transmitter/receiver pairs are utilized in close proximity to each other and share a common transmission medium.
However, among other things, such frequency division multiple access schemes suffer from the deficiency that unique electronic circuitry, corresponding to the unique frequency band for each product, must be employed in each product in order to allow the products to operate at different frequencies. Naturally, this increases the cost of such products and complicates the manufacturing process by unnecessarily complicating inventory control and production flow during manufacturing, etc. Furthermore, when such products are mass produced it is simply impossible to ensure that a unique communication channel has been assigned to every one of the products because of the limited frequency range which has been allocated to such uses. Accordingly, while steps can be taken to minimize the risk of signal confusion, some frequency channel duplication inevitably occurs. Naturally, this gives rise to the possibility that users of a given product will interfere with one another. All of these deficiencies limit the use of frequency division multiple access schemes in simplex mode communication systems.
In contexts other than simplex mode communication systems time division multiple access schemes have also been employed with some degree of success. Time division multiple access schemes are often used in conjunction with frequency division multiple access schemes to further reduce the possibility of conflicts between users of a given product. Time division multiple access schemes, however, avoid some of the above-described deficiencies of frequency division multiple access schemes by ensuring that transmitted messages are separated in time. Restated, time division multiple access schemes allow multiple transmitter/receiver pairs to utilize unique time slots for the transfer of information, even if the transmitter/receiver pairs operate in the same frequency band.
Some of the time division schemes currently in use include time division multiple access ("TDMA"), ALOHA (either pure or slotted) and carrier-sense multiple access with collision-detection ("CSMA/CD"). Each of the schemes has a number of variations. However, all of these multiple access schemes have proven to be difficult and costly, and in some cases impossible, to implement in simplex mode communication systems. For example, TDMA allows for communication between multiple transmitter/receivers over a shared frequency band of a transmission medium by allocating synchronized time frames to each of the transmitter/receiver pairs. Thus, in a TDMA system, a given time frame is divided into individual time-slots and each transmitter is pre-assigned unique time-slots in which to transmit information to its receiver. However, it is necessary for the various receivers and transmitters of the system to be synchronized with one another for the system to operate effectively. Thus, TDMA requires a centralized system manager to pre-assign time-slots, communicate those assignments to the various transmitter/receivers and monitor the synchronization between the various components of the system during operation. This, naturally, leads to an undesirable increase in network complexity and expense. Alternatively, known handshaking techniques can be used to coordinate message transmission between transmitter/receiver pairs. However, such a system cannot be implemented in a simplex mode communication system because it requires the transfer of information between transmitter/receiver pairs in both directions.
A pure ALOHA multiple access protocol avoids the need for a system manager by allowing transmitters to transfer information to respective receivers at random times regardless of whether or not other transmitters are attempting to do the same. Using ALOHA, however, each transmitter must monitor the transmission medium for signal collisions between its own signal transmissions and signal transmissions from other transmitters. When such collisions occur, the transmitters must wait a random period of time and retransmit the collided messages while, again, monitoring the transmission medium for another signal collision. This process is repeated until transmission of the messages is successfully completed, i.e. no collision between these messages and any other messages occurs. However, since the pure ALOHA protocol does not even attempt to avoid signal collisions, there is no guarantee that any given signal will be successfully transmitted. Further, the pure ALOHA protocol suffers from the deficiencies of low throughput rates and instability. To overcome these deficiencies a variation of the pure ALOHA protocol known as slotted ALOHA has been developed. However, the slotted ALOHA protocol cannot be implemented in an asynchronous simplex mode communication system because the receivers of a slotted ALOHA system must send message-reception acknowledgments to the transmitters (i.e., handshaking) and because the transmitters must be synchronized with one another.
Yet another attempt to improve the related art time division multiple access schemes is CSMA/CD. CSMA/CD attempts to avoid signal collisions by allowing transmitters to transmit data messages only after monitoring the transmission medium to ensure that the transmission medium is not currently in use by another transmitter. If the transmission medium is in use, the transmitter must wait until the transmission medium is clear before transmitting a message. However, one major deficiency of CSMA/CD systems is that, between the time that a transmitter finishes checking whether the transmission medium is free, and the time that it begins transmitting, another transmitter can begin transmitting another message. The result is a collision of the two transmitted signals. If such a collision occurs, the transmitters broadcasting the colliding signals stop transmitting immediately and wait a random time period before attempting retransmission. This process is repeated until both of the transmissions are successfully completed. One of the primary deficiencies of the CSMA/CD protocol is that the signal-collision detection and back-off algorithms utilized to effectuate signal transmissions necessarily results in a significant increase in the cost and hardware complexity of the system. Additionally, the various transmitters of a given system must be synchronized with one another to operate effectively.
Therefore, there remains a need in the art for a multiple access protocol for use in simplex communication systems which overcomes the aforementioned deficiencies of the related art by dispensing with the need for synchronization between the transmitters of a multiple-transmitter system while guaranteeing successful reception of all transmitted data messages.
There remains an additional need in the art for a multiple access protocol for use in simplex mode communication systems which overcomes the aforementioned deficiencies of the related art by avoiding the need to synchronize the various components of the system while reducing the complexity of the hardware needed to implement the protocol.
There remains a further need in the art for a multiple-access protocol for use with simplex mode communication systems which overcomes the aforementioned deficiencies of the prior art by accommodating a significant number of transmitter/receiver pairs on a single frequency band while offering a significant throughput rate and eliminating the possibility of signal collision among the various transmitters of the system.